As is well known to both the commercial and sport sailor, it is frequently desirable that a sail boat have the capacity to steer itself, thereby relieving the helmsman for other duties or rest. Such occasions might arise during lengthy ocean passages, when sailing with shorthanded crew, or, particularly, when sailing single-handed. In all such circumstances, it is essential that the self-steering device have complete control of the helm to prevent undesired motions of the sail boat or deviations from a preset course.
Numerous efforts have been made to develop self-steering systems for sail boats, for the general purpose described as well as for other purposes. One category of such mechanisms, for example, has the purpose of maintaining a boat on a given course with a fixed compass heading. As mechanisms in this category generally require auxiliary power responsive to control by some compass device, they are of no interest to the present invention which is specifically directed to a wind powered device designed to hold the boat on a selected heading or course relative to the apparent wind direction. Another category of self-steering mechanisms has the limited purpose of providing direction to model yachts, and is likewise of no interest to the present invention because the self-steering arrangements for model yachts are not effective in performing the functions required for full-size boats.
In the particular category of self-steering mechanisms for sports sail boats, some fairly effective and reliable systems have been developed in the past decade for achieving self-steering relative to the apparent wind direction. Such systems generally employ a windvane to measure the apparent wind direction, and use the wind pressure on the windvane, when the ship wanders off course, to drive a steering device that will steer the ship back to the desired course. In such systems, the steering device can be the ship's tiller, an auxiliary rudder or trim tab, a servo blade for amplifying power, or like means, depending upon the particular system used. Self-steering mechanisms of this general type, wherein windvanes are employed to sense apparent wind direction, are disclosed in Gianoli U.S. Pat. Nos. 3,180,298 and 3,319,594, Ross-Clunis U.S. Pat. No. 3,678,878 and Saye U.S. Pat. No. 3,765,361.
Although effective to a degree, known self-steering mechanisms of the type described have not proved to be entirely satisfactory in use. Reasons for this result have been sought in theoretical and experimental studies. Thus, assuming that yaw or sway is of primary interest in determining the directional stability criterion for a sailing vessel (and that roll, pitch, surge or heave have a minimal effect), it can be determined that the stability criterion is a non-linear function of the forward speed of the vessel. Further, if it is assumed that there is a time lag between a wind powered input signal to the windvane, and the resulting corrective deflection of the rudder, it can be determined that the time lag in the self-steering system is dependent on the velocity of the water flowing over the auxiliary rudder (trim tab); also, that the time lag will be greatest when the water velocity is small. The conclusion, therefore, which has been verified by experimental work, is that a truly successful self-steering mechanism must yield an adjustable, non-linear response to the sensing of apparent wind. Upon analysis, it has been further determined that known self-steering systems are defective in the desired capacity of providing an adjustable, non-linear response for control of both the stability criterion and the time lag of the system, particularly in a simple, small, lightweight mechanism designed for use in sport sailing operations.